Porous media tunnel burner

ABSTRACT

A porous media burner comprises an outer casing of impermeable material separated by a gas distributing space from an inner hollow core, open at one end, of a porous medium. The fuel mixture enters the gas distributing space between the casing and the core and then passes through the core to the hollow space of the core.

limited States Patent 1191 Westlake et al. 1 1 May 15, 1973 [54] POROUSMEDIA TUNNEL BURNER [56] References Cited UNITED STATES PATENTS [751Invemms: Donald wesuake Flee? Kenneth 3,191,659 6/1965 Weiss ..43l/328C0195, Famborough, both of 3,119,439 1/1964 Weiss ..431 32s gland3,425,675 2/1969 Twine ..43l/328 1,223,308 4/l9l7 Bone et al, ..43l/328[73] Assignee: Shell Oil Company, New York,N,Y 3,208,247 9/1965Weiletal. ..43l/328 Filed: July 1971 Primary ExaminerCarroll B. Dority,Jr. [21] Appl' No; 161,109 Attorney-Harold L. Denkler et al.

I [57] ABSTRACT A orous media burner com rises an outer casin of P t P P8 [30] Foreign Application "only Da 8 impermeable material separated bya gas distributing July 13, 1970 Great Britain ..33,835/70 space from aninner hollow core, open at one end, of a 1 porous medium. The fuelmixture enters the gas dis- [52] U.S. Cl ..43l/328 tributing spacebetween the casing and the core and [51] Int. Cl.., ..F23d 13/12 thenpasses through the core to the hollow space of [58] Field of Search ..431/158, 328 the core.

8 Claims, 3 Drawing Figures PATENTED HAY] 51973 Dondld West/eke KennethF. Coles IN VE N TORS 1 POROUS MEDIA TUNNEL BURNER BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates to the fieldof burners and, more particularly, to a tunnel burner with a porousmedia stabilizer for use in heating appliances.

2. Description of the Prior Art Porous media burners are suitable for alarge variety of applications in the heat treatment, drying andprocessing field, such as ceramics, domestic heating and cooking, roadheating, chemicals processing, textiles, paints, foods, paper, ferrousand non-ferrous metals, glasses, refractory materials, cement and lime.

In porous media combustion systems an unburnt gaseous fuel-oxidantmixture is fed to the upstream side of a porous solid and combustion orreaction takes place near the downstream surface. A variety of porousstructures and solid materials offers a variety of reaction zones:combustion may take place in the gas phase above the porous media,inside the pores themselves or on the pore surface within a porousmedium (which may or may not be catalytic).

Heat-treating processes continue to need faster heating rates andprecision in heat application; such requirements can be met by takingadvantage of the high heat transfer flux which is potentially availablefrom a high temperature high velocity gas stream.

In this connection porous media combustion systems may be suitably usedbecause of the possibility of high heat transfer rates, silent operationand clean combustion over a range of gas/oxidant ratios.

SUMMARY OF THE INVENTION Applicant has found a porous media stabilizerwhich is able to withstand severe operation conditions and to fulfillhigh requirements concerning temperature distribution and thepredetermined control thereof and which is particularly suitable forincorporation in a high intensity tunnel burner.

According to the invention a porous media tunnel burner comprises aclosed outer casing of impermeable material and connected therewith aninner hollow core of a porous medium open at one end. The inner core isspaced from the outer casing whereby a gas distributing space is formedbetween the casing and the core. This space is in communication with aninlet in the casing for a combustible gas mixture. The arrangement issuch that the mixture can pass through the core from the gasdistributing space to the hollow space of the core.

A main advantage of the burner according to the invention is that duringoperation this burner is selfcooling because heat which tends to leakthrough the porous core is met by the incoming cold gas stream which inturn is preheated. The casing consequently is a cold wall and noexternal cooling is required.

Further, the porous core presents an inlet port area uniformly spreadover substantially the whole of the hollow space of the core which mayconstitute the combustion space. This large exit surface area results ina low pressure drop across the core and a laminar exit of the gas fromthe pores of the core. This produces a stabilizing effect, whilst a highcombustion intensity is obtained per unit of surface volume of thecombustion space. As a consequence thereof, for a given output a lowermixture pressure is required than in conventional high intensity burnersand a smaller combustion space is needed than that of the conventionalhigh intensity burners.

Moreover, the noise level is relatively low and the stable combustioncauses no undesirable vibration.

According to the invention in a particular embodiment, in a porous mediaburner the casing and the hollow core are cylindrical and coaxial, thecore being connected to the casing at or near its open end, a centralaxial inlet being provided in the casing at the end facing the closedend of the hollow core.

This very compact burner can easily be installed in ovens or likecombustion appliances, whilst the central inlet of the mixture ensuresan even distribution over the symmetrical annular gas distributingspace.

Although the burner can be made of any material having a suitably lowthermal conductivity the use of a refractory material can be veryattractive. According to a preferred embodiment of the invention thecasing and the core are made of different gradings of a high alumina orstabilized zirconia aggregate. The casing and the core part can beconnected by sintering. The casing and the core part may consist of thesame composition or at least compatible compositions.

The alumina is shock-resistant and withstands high temperatures, iserosion-resistant and is of such a mechanical strength that duringsevere operating conditions no damage of the elements is to be feared. Amethod of manufacturing such ceramic elements of different porositycomprises the steps of separating a refractory capable of being sinteredinto a plurality of grain size gradings by sieving, casting a permeablecore from one or more gradings of refractory with a predeterminedquantity of cement and water-in a frame-type mould, allowing the core tobe air-cured, sintering the core by heat treatment, cooling the sinteredcore, casting a substantially non-permeable refractory casing in a mouldaround the core from a dense grading of the refractory with cement andwater and allowing it to be air-cured, sintering the casing around thecore by heat treatment and cooling the so formed porous ceramic element.

Such elements having a core of relatively high permeability, laterallysurrounded by a layer of substantially no permeability being the samematerial but of different grain size grading than the core, offer theadvantage of being substantially free from lateral leakage and able towithstand thermal stresses. Moreover, the problems in connection withmetal casings, which gave rise to operational troubles in conventionalradiant heating appliances, are avoided. A major advantage is thatcomplete predetermined control of combustion is possible by properselection of the grain size gradings.

The heat treatment of the complete element may be carried out duringabout 12 hours including a soaking period up to 3 hours at a temperatureof about l,600C. In the step comprising the manufacture of the porouscore the temperature is, preferably, about 1,700C, which is maintainedduring a period of up to 4 hours.

The separation of the basic material into different gradings may beeffected by passing it through successive sieves of ranges of 10-18,18-25, 25-36 British Standard Sieve, respectively; also gradings of awider range may be used, i.e., the range of l8-36 British StandardSieve.

The basic material for the core part may also be of finer gradings thanindicated above; gradings of a fineness approaching the powdery statemay be applied. When manufacturing blocks wherein very fine gradings forthe basic material in the core part are used, it is ad visable to addtogether with the cement supplementary material to the core which duringsintering will be burnt out, so as to ensure a satisfactory permeabilityof the core part.

The alumina cement preferably comprises refractory passing through a 200mesh B.S. sieve and then mixed up with about percent water for use as abonding material in the manufacture of the elements. The cast materialwill then be air-cured during a period of at least hours.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF A PREFERRED EMBODIMENTIn the drawings 1 designates a burner casing of impermeable refractorymaterial provided with a core 2 made of a porous refractory material. Agasdistributing space 3 is present between the casing 1 and the core 2and extends over substantially the complete outer surface of the core 2,which is a hollow body provided with a central combustion space 4. Thecore 2 is closed at one end by a seal cover 5. The distribution space 3is in communication with an inlet port 6 in the casing 5 for the supplyof a combustible mixture. The combustion space 4 opens at the endopposite the seal cover 5, into an outlet port 7 provided within nozzlepart 8.

In FIG. 2 an embodiment having the shape of a rectangular prism isshown, wherein the casing is laterally closed by end covers 9.

In FIG. 3 a cylindrical embodiment is shown, the distributing spacebeing in the form of an annular gap 3' between a cylindrical outercasing 1' and cylindrical inner core 2.

In operation the fuel (e.g., natural gas or LPG) air or oxidant mixtureenters the inlet pipe 6 at the rear of the burner and passes down thegap 3 between the outer body and inner body (the casing l and the core2, respectively) and then through the porous medium. Combustion takesplace near the inner surface of the porous medium; the combustion gasespass out through the outlet 7 of nozzle portion 8.

In the embodiment of FIG. 2, as an alternative the casing also may bemade of one piece instead of being provided with end covers 9. Also aplurality of inlet pipes 6 may be provided, distributed over the lengthof the burner.

In both embodiments, as a further alternative a separate nozzle part 8,which e.g., is made of zirconia, is not a strict necessity, as this partmay also be formed as an integral part of the casing or by extending thecore 2 till the end outlet face of the burner. The outlet 7 may also beof the same width (or diameter) as the combustion space 4 and both maybe either circular or rectangular. Further, instead of closing the endof the core 2 by a seal cover 5, the core may be one piece, the outerwall at the closed end thereof then may be streamlined and have asomewhat greater wall thickness than the remaining part of the core. Theusual wall thickness of the core, depending on the design and capacityof the burner, preferably is between l/r inch (6mm) and 2 inches (50mm).

The burner is suitable for operation stoichimetric mixtures as well aswith non-stoichiometric mixtures, such as by partial combustion forcreating a reducing gas, but also for creating oxidizing atmospheres, byoperating with a surplus of oxidant.

The burner according to the invention is particularly suitable for useas a tunnel burner and fulfils immediate requirements in the rapidheating of metal field for a high velocity, high mass throughput burner(e.g., I00 kg/h fuel). However, the burner structure according to theinvention can be designed to produce any shape or size of high intensityburner.

We claim as our invention:

1. A porous media burner comprising:

an impermeable casing having inlet and outlet ports therein;

a hollow core having a wall composed of a permeable, porous mediumconnected to the casing, the hollow core having an opening therein andbeing positioned within the casing with the opening in communicationwith the outlet port of the casing;

the casing and the hollow core being composed of a refractory materialof substantially the same composition;

the hollow core being spaced from the casing about a substantial portionof the outer surface area of the core whereby a gas distributing spaceis defined between the hollow core and the casing;

the gas distributing space being in communication with the inlet port ofthe casing;

whereby a combustible gas mixture may flow through the inlet port intothe gas distributing space, pass through the permeable wall of thehollow core from the gas distributing space to the hollow space of thecore, and then exit from the casing through the outlet port.

2. The porous media burner of claim 1 wherein the casing and the hollowcore are cylindrical and coaxial, the opening in the core and the outletport of the casing being adjacent one end of the cylindrical casing andthe inlet port being adjacent the opposite end of the cylindricalcasing.

3. The porous media burner of claim 1, wherein the refractory materialof the casing and core, respectively, comprises first and secondgradings of a high alumina aggregate, the first grading being more densethan the second.

4. The porous media burner of claim 1, wherein the refractory materialof the casing and core, respectively,

comprises first and second gradings of zirconia-based aggregates, thefirst grading being more dense than the second.

5. The porous media burner of claim 1 wherein the casing and the coreare connected by sintering.

6. The porous media burner of claim 1 wherein the wall thickness of thehollow core is between one-fourth inch and 2 inches.

7. The porous media burner of claim 1 including a nozzle part positionedin the outlet port of the casing in communication with the opening inthe hollow core.

8. The porous media burner of claim 7 wherein the nozzle part consistsof a zirconia aggregate and the casing and core consists of a aluminaaggregate.

1. A porous media burner comprising: an impermeable casing having inletand outlet ports thereiN; a hollow core having a wall composed of apermeable, porous medium connected to the casing, the hollow core havingan opening therein and being positioned within the casing with theopening in communication with the outlet port of the casing; the casingand the hollow core being composed of a refractory material ofsubstantially the same composition; the hollow core being spaced fromthe casing about a substantial portion of the outer surface area of thecore whereby a gas distributing space is defined between the hollow coreand the casing; the gas distributing space being in communication withthe inlet port of the casing; whereby a combustible gas mixture may flowthrough the inlet port into the gas distributing space, pass through thepermeable wall of the hollow core from the gas distributing space to thehollow space of the core, and then exit from the casing through theoutlet port.
 2. The porous media burner of claim 1 wherein the casingand the hollow core are cylindrical and coaxial, the opening in the coreand the outlet port of the casing being adjacent one end of thecylindrical casing and the inlet port being adjacent the opposite end ofthe cylindrical casing.
 3. The porous media burner of claim 1, whereinthe refractory material of the casing and core, respectively, comprisesfirst and second gradings of a high alumina aggregate, the first gradingbeing more dense than the second.
 4. The porous media burner of claim 1,wherein the refractory material of the casing and core, respectively,comprises first and second gradings of zirconia-based aggregates, thefirst grading being more dense than the second.
 5. The porous mediaburner of claim 1 wherein the casing and the core are connected bysintering.
 6. The porous media burner of claim 1 wherein the wallthickness of the hollow core is between one-fourth inch and 2 inches. 7.The porous media burner of claim 1 including a nozzle part positioned inthe outlet port of the casing in communication with the opening in thehollow core.
 8. The porous media burner of claim 7 wherein the nozzlepart consists of a zirconia aggregate and the casing and core consistsof a alumina aggregate.